Maize is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop high yielding maize hybrids that are agronomically sound based on stable inbred lines. The reasons for this goal are obvious: to maximise the amount of grain produced with the inputs used and minimise susceptibility of the crop to environmental stresses. To accomplish that goal, the maize breeder must select and develop superior inbred parental lines for producing hybrids. This requires identification and selection of genetically unique individuals which in a segregated population occur as the result of a combination of crossover events plus the independent assortment of specific combinations of alleles at many gene loci which results in specific genotypes.
Several different strategies have been used to increase the grain yield of maize. The most generally utilised approach is to select for increased yield per se. Another approach is to try to preserve the inherent yield potential by reducing losses that occur due to disease and insect pests and following exposure to environmental stress. Thus, in many commercial programmes, breeders select for disease and insect resistance and tolerance of drought stress.
Another approach to increasing yields has been to increase the number of plants per unit area, typically per hectare or acre, or the “plant density”. It has been stated that in the 1930s, maize farmers in the USA planted 10,000 plants per acre whereas by 1998, the farmers were planting 20,000 to 30,000 plants per acre. With increasing plant density maize plants tend to grow taller and become more susceptible to lodging. Thus, to develop commercial products adapted to higher planting rates, breeders have selected for resistance to stalk and root lodging.
Maize has shown a limited ability to increase yields of both dry matter and grain as plant densities are increased. Mock & Pearce (1975) described an optimum environment to produce maximum yields as including, among other factors, high plant densities and narrow rows. High plant densities and narrow rows permit increased leaf area index (LAI (Leaf Area Index)=leaf area per unit land area) allowing interception of more of the light energy reaching the earths surface.
Presently available maize plant morphologies (prolific phenotypes) often enable a maize plant to produce a second ear of harvestable grain but only if plant densities are below normal for typical growing conditions. Earley et al (1974) noted the yield of grain per plant was not limited by the lack of potential ears but by the failure of one or more of the earshoots to develop into sizeable ears. Results of experiments by Harris et al (1976) indicated that the certain lower earshoots abort because they reach the silking stage in poor synchrony with upper ear shoots. This was confirmed by studies by Sorrels et al (1979). As plant densities are increased, the incidence of plants with two ears becomes progressively lower. Prine (1971) concluded that under high plant densities competition for light during the critical silking period resulted in sizeable reductions in grain yield. Moreover, with increasing plant densities, the incidence of plants with multiple ears decreases more rapidly than the total vegetative matter per plant. Thus, obtaining a morphology which routinely produces multiple ears with harvestable grain, on a plant with equal or less total dry matter yield, would be an advantageous alternative for increasing yield. However, under very high plant densities, most of the plants are barren even though the total dry matter yield per unit area of land increases with increasing plant densities. Although grain yield is reduced at very high populations, the result is encouraging. This population response of the plant suggests that with appropriate technology to enable more efficient conversion of photosynthate into grain, the grain yielding potential of maize on a per acre basis could be further increased as well.
Attempts to improve the yield of maize by applying a dwarfing strategy used effectively in wheat and rice have not to date been successful. A widely used gene in maize is the brachytic 2 mutant. The lower internodes of brachytic 2 dwarfs are much shorter than in normal maize. Leng & Ross (1979) found that at comparable planting rates brachytic 2 dwarf hybrids showed better standability than their normal counterparts but yielded less. Subsequently, Pendelton & Sief (1961) found that the yield deficiency of brachytic 2 dwarfs could not be overcome by closer row spacings or higher plant populations. A more recent effort by Castiglioni et al (1991) studied the effects of the brachytic 2 gene following introduction into seven maize varieties. The introduction of the gene significantly reduced plant and ear height, plant lodging also decreased significantly. However, as with previous studies, grain yields declined compared to normal counterparts, and there was a measured reduction in the degree of prolificacy.
Research by plant physiologists has identified the magnitude of the supply of photosynthate available to convert into grain (also called “source” capacity), and the capacity to convert that supply into grain (also called “sink” capacity), as potentially limiting factors in maize yield. Tollenar (1977) summarised those reports by concluding that grain yield is limited by sink size in most temperate and subtropical maize growing environments, the exception being the northern areas of North America where the source appears to be limiting. The source limitations could be overcome by increasing LAI through high density plantings to the point when sink size ultimately becomes the limiting factor in grain yield, if not for the fact that eventually high plant density suppresses expression of prolificacy and reduces grain yield. Anderson et al (1984) conducting N rate studies confirm the results of Harris et al (1976) to the effect that reproductive sink size limits the yield of non-prolific hybrids. When sink size is the limiting factor, increasing the number of potential energy sinks (ear sites) could be achieved through improved multiple ear capability (prolificacy). Traditionally, Tollenar (1977) noted that increasing the amount of photosynthate to the ear during flowering would also increase yields. Thus, developing maize plants having improved multiple ear formation and increased available photosynthate during flowering would be desirable to improve the maize crop.